Method and installation for synthesising methanol from hydrogen, carbon monoxide and carbon dioxide under pressure

ABSTRACT

A process and a device are for the methanol synthesis from hydrogen, carbon monoxide and carbon dioxide under pressure, in particular for increasing the yield of processes already in use. Desulphurized natural gas is charged in a reformer and the synthesis gas is subsequently admitted to a methanol synthesis. There are the advantages that a favorable possibility for refitting existing plants is provided, and synthesis gas from external sources can be used. This is accomplished in the process in that after passing through the reformer, a side stream from the synthesis gas stream is supplied to a methanol pre-reactor. The methanol produced in the pre-reactor is supplied to the methanol stream exiting from the methanol synthesis of the main stream. A stream of synthesis gas non-reacted in the methanol pre-reactor is recycled into the main stream upstream of the methanol synthesis. An additional synthesis gas compensating the incurred loss is simultaneously charged within the zone of said feed into the main stream.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicant claims priorities under 35 U.S.C. §119 of German ApplicationNos. 199 42 559.0, filed: Sep. 7, 1999, 199 51 137.3, filed: Oct. 23,1999, and 100 00 280.3, filed: Jan. 7, 2000. Applicant also claimspriority under 35 U.S.C. §120 of PCT/EP00/06488, filed: Jul. 8, 2000.The international application under PCT article 21(2) was not publishedin English.

The invention relates to a process for the methanol synthesis fromhydrogen, carbon monoxide and carbon dioxide under pressure, inparticular for increasing the yield of processes already in use, wherebydesulphurized natural gas is charged in a reformer and the synthesis gasis subsequently admitted to a methanol synthesis.

A number of devices or processes for the catalytic methanol synthesisare known, whereby the following documents are cited here as examples ofthe abundance of solutions: DE 21 17 060; De 25 29 591; DE 32 20 995; DE35 18 362; U.S. Pat. No. 2,904,575; and DE 41 00 632.

A methanol production plant is normally operated in conjunction with aplant for producing synthesis gas, whereby both plants are dimensionedin such a way that the synthesis gas produced exactly covers therequirements of the methanol-producing catalyst of the synthesis gascirculation and in particular is stoichiometrically composed based onthe following reactions, whereby only two of said reactions are linearlydependent upon each other: $\begin{matrix}{{CO} + {2\quad {H_{2}{CH}_{3}}{OH}}\quad - {90.84\quad {{kJ}/{mol}}}} & (1) \\{{{CO}_{2} + {H_{2}{CO}} + {H_{2}O} + {41.20\quad {{kJ}/{mol}}}}\quad} & (2) \\{{{CO}_{2} + {3\quad {H_{2}{CH}_{3}}{OH}} + {H_{2}O} - {49.64\quad {{kJ}/{mol}}}}\quad} & (3)\end{matrix}$

According to the above reaction equations, the following applies to astoichiometric synthesis gas: $\begin{matrix}{\zeta = {\frac{{{}_{}^{}{}_{}^{}} - {{}_{}^{}{}_{}^{}}}{\quad^{C}{CO}\quad  +^{C}{CO}_{2}} = 2}} & (4)\end{matrix}$

whereby c_(i) is the gas concentration of the respective startingmaterials based on moles.

The subscript “i” represents the individual compounds. Alternatively,c_(i) can be written as set forth above, thus “c_(i)” can refer to:^(c)H₂, or ^(c)CO₂, or ^(c)CO, or ^(c)CO₂.

Such a synthesis gas is usually produced in one single production linein a primary reformer, or together with a secondary reformer or insimilar gas production equipment.

It is difficult to refit such a plant at a later time if existingproduction capacities have to be expanded. Generally, another productionplant has to be built in such a case based on the layout of the old one,and no synergy effects can be expected. Furthermore, tying-in synthesisgas from external sources, which is often available at low cost whenother larger plants are shut down, poses problems in that because ofdifferent compositions of the gas, which in most cases is an excessivecomponent of carbon-containing compounds (“C-component”), such gascannot be used for the production of methanol without further processingrequiring substantial expenditure.

Therefore, the problem of the invention is to expand the known processby overcoming the drawbacks that have become known so far, creating goodpossibilities for later refitting of existing plants, and to make itpossible to employ synthesis gas from other sources.

With a process of the type specified above, said problem is solvedaccording to the invention in that after passing through the reformer, aside stream from the stream of synthesis gas is supplied to a methanolpre-reactor; the methanol produced in the pre-reactor is fed into themethanol stream exiting from the methanol synthesis of the main stream;and a stream of synthesis gas that has not been reacted in the methanolpre-reactor is recycled into the main stream prior to the methanolsynthesis, whereby additional synthesis gas compensating the incurredloss is simultaneously charged within the zone where the non-reactedsynthesis gas is recycled.

The methanol pre-reactor employed in the process, as well as the devicesusually employed for conditioning the synthesis gas, as well as for thecondensation and separation of the methanol produced can be refitted ata later time, whereby it is possible in this way to increase theproduction capacity up to 57% based on the capacity of the oldinstallation. Of course, this concept is not limited to the laterrefitting of old plants, but it can be advantageously taken into accountin the new conception as well.

Further developments of the invention follow from the dependent claims.Provision can be made in this conjunction for using as the additionalsynthesis gas a synthesis gas from external sources, i.e. from aseparate synthesis gas source, or a synthesis gas that is withdrawn fromthe stream of natural gas as a bypass and then passed via an “autotherm”reformer, or a synthesis gas that originates from another synthesis gasproduction.

The foreign synthesis gas may originate in this connection from acombined “autotherm” reformer (also referred to as “CAR”), or from areactor for partial oxidation, which can be refitted at a later time andis substantially more favorable in terms of construction than theapparatuses usually employed for producing synthesis gas with an exactly“fitting” gas composition. In this case, it becomes possible alsojointly use a portion of the exhaust gas of the methanol productionplant, which otherwise would be usable only as firing gas, as anadditional gas charged in an “autotherm” reformer, or in a combined“autotherm” reformer.

It may be advantageous in this conjunction if the additional synthesisgas compensating the loss and fed into the main stream of the synthesisgas prior to the methanol synthesis, is withdrawn from a combined“autotherm” reformer and/or from a reactor for partial oxidation, orfrom another synthesis gas production, for which provision is madeaccording to the invention as well.

According to another further development of the invention, the off-heatthat has to be dissipated from the methanol pre-reactor during thecooling of the methanol synthesis gas mixture, is exploited foroperating an absorption refrigeration machine. The produced cold is usedfor cooling the synthesis gas exiting from the reformers before it iscompressed, so that compression energy is saved in this way.

The saved compression energy can be exploited according to the inventionfor compressing the synthesis gas obtained from external sources, or theadditional synthesis gas produced in refitted reformers. Furthermore,the produced cold can be used for condensing out more methanoldownstream of the methanol reactors. If the objective is to primarilysave investment costs, and if the energy costs are low, it may beeconomical also to dispense with any exploitation of the off-heat and toemploy instead of an absorption refrigeration machine a conventionalrefrigeration machine operated with, for example ammonia as therefrigerant.

With the mixture of the foreign synthesis gas and the synthesis gasobtained from the methanol pre-reactor after the methanol produced hasbeen separated, it is necessary to ensure that the mixture approximatelycorresponding with the composition of the synthesis gas that has beenoriginally branched off, fully covers the conditions for the existingmethanol circulation, and that the original synthesis can be operatedwithin the framework of the catalyst conditions. In order to assurethis, at least the following must apply to the molar concentrations ofthe foreign synthesis gas: 0.8≦ζ≦4. An improvement is obtained if thefollowing applies additionally: 1≦ζ≦2.5. The greater the proportion ofthe withdrawn side stream and the methanol produced in the pre-reactor,the more careful hone has to make sure that the characteristic ζ of theforeign synthesis gas approaches number 2.

A further development of the invention consists in that a mixture of H₂and CO₂ is used as the additional synthesis gas, whereby the CO₂ presentin the mixture originates from the flue gas of a firing installation orfrom the exhaust gas of a CO₂ washing operation in an ammonia plant(FIG. 2).

Using in the additional synthesis gas the CO₂ originating from acombustion process offers the great advantage that said CO₂ does nothave to be discharged into the atmosphere, but is employed as intendedfor the synthesis.

Provision is made in this connection for the further development thatthe CO₂ from a flue gas purification plant originates from a firingoperation, for example from the firing of the primary reformer.

Said procedure offers the additional benefit that the climaticallyactive CO₂ is introduced into the substance circulation, so that a feecharged, for example for “hothouse emissions” needs no longer to bepaid, which renders the procedure substantially more economical.

In addition to using the CO₂from a combustion plant, such CO₂ mayoriginate from the CO₂ washing operation of an NH₃ plant as well. Suchan ammonia plant is not explained in greater detail in the following.

For solving the problem specified above, the invention makes provisionalso for a device for the methanol synthesis, in particular a device forcarrying out the process according to any one of the preceding claims,such a device comprising a natural gas supply (or feed) line leading toa reformer, and a methanol synthesis connected downstream, whereby sucha device as defined by the invention is characterized by the following:

a branched-off line for a side stream of the synthesis gas exiting fromthe secondary reformer;

a methanol pre-reactor in the side stream;

a methanol feed line leading to the methanol main stream exiting fromthe methanol synthesis;

a recycling line recycling non-reacted synthesis gas from the methanolpre-reactor into the synthesis gas main stream upstream of the methanolsynthesis; as well as

a feed line upstream of the methanol synthesis for feeding synthesis gascompensating losses.

As a further development of the invention, the device as defined by theinvention is characterized in that an “autotherm” reformer is connectedin parallel with the reformer, whereby the exiting synthesis gas is atleast partly used as synthesis gas compensating losses.

According to the invention, provision can be made also for a flue gaswashing operation for the flue gas exiting from the primary reformer, aswell as for a CO₂feed line for supplying the foreign or additional gas,as well as for a feed line for feeding foreign H₂ (FIG. 2), whereby itmay be useful if provision is made for a supply line for feeding foreignCO₂ from a CO₂ washing operation in the NH₃ plant, as well as for asupply line for feeding foreign H₂, for which lines provision is made infurther development of the invention as well.

Further details, advantages and features of the invention are containedin the following description and shown in the drawing. The drawing showsthe following in a highly simplified manner:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a device as defined by the invention; and

FIGS. 2 and 3 show modified examples of the block diagram in acomparative representation.

The following is by way of example a description of the laterrefittability of a device 1, whereby the mode of operation is describedat the same time.

A device 1 for producing methanol, which is already in operation andcomprises a primary reformer 2, a secondary reformer 3 and a methanolsynthesis 4, is additionally equipped at a later time with a methanolpre-reactor 5 and an “autotherm” reformer 6. The material charged is thedesulphurized natural gas 7, which is passed into both the primaryreformer 2 and the “autotherm” reformer 6, whereby other startingmaterials—which are not shown here—such as, for example steam and oxygenare added as well. A crude synthesis gas is produced from said startingmaterials in the primary reformer 2. Said crude synthesis gassubstantially consists of hydrogen, carbon monoxide, carbon dioxide, andnon-reacted steam and natural gas. The residual natural gas is reactedwith oxygen in the secondary reformer 3 to hydrogen and carbon monoxide.The conditions in the primary reformer 2 and in the secondary reformer 3are adjusted in this connection in such a way that a stoichiometricsynthesis gas 8 according to the equations (1) to (4) is produced. Aside stream 9 is branched off from said synthesis gas and supplied tothe methanol pre-reactor 5. A portion of said synthesis gas is reactedin the methanol pre-reactor 5, condensed out, separated and dischargedas the additional methanol 10. The remaining below-stoichiometricsynthesis gas 11 is recycled, i.e. admixed again to the original mainstream of the synthesis gas.

In order to avoid the formation of a below-stoichiometric mixture, theabove-stoichiometric synthesis gas 13 is admixed to the main stream 12of the synthesis gas as well. Said above-stoichiometric synthesis gas 13consists of the foreign synthesis gas 14 and/or the additional synthesis15, which has been produced only for that purpose in the refitted“autotherm” reformer 6, whereby the “autotherm” reformer 6 has to beunderstood as being an example only.

The stoichiometric synthesis gas 16 is formed with the admixtures addedto the synthesis gas main stream 12. Said synthesis gas 16 has acomposition equivalent to the stoichiometric synthesis gas 8. This meansthat a methanol 17 is produced in the conventional manner in the oldmethanol synthesis 4 which, together with the additional methanol 10,forms the new methanol product 18.

In modification of the device according to FIG. 1, FIG. 2 shows a devicewhere foreign CO₂ and foreign H₂ are fed via the lines 19 and,respectively, 20 into the line 14 feeding the foreign synthesis gas. TheCO₂ of the line 19 originates from a flue gas washing operation 21,which receives the flue gas from the primary reformer 2 via the supplyline 22. The flue gas is purified in the flue gas washing plant 21 andthe latter then feeds the CO₂ into the line 19, and feeds the exhaustgas into a smoke stack 23 indicated only symbolically. Said exhaust gasis obviously free of CO₂.

FIG. 3 shows the alternative, by which foreign CO₂ is supplied via theline 19 a to the line 14 for the foreign synthesis gas, whereby saidforeign CO₂ originates from the CO₂ washing operation of an ammoniaplant not shown here in detail.

Foreign H₂ is supplied to the system via a line 20 a in the present caseas well.

It is obvious particularly in connection with the variation according toFIG. 2 that completely closed CO₂ circuits are possible, i.e. theseplants operate in such a way that no CO₂ has to be discharged into theenvironment.

The following numerical example in table 1 serves for furtherillustration. It is assumed in this connection that an old plant forproducing 1000 tons/day methanol exists and that its production capacityis to be expanded by about 35%. The numbers relate to the referencenumerals in FIG. 1.

TABLE 1 Number 8 9 10 11 12 13 16 17 18 Quantity [kmol/h] 5147 3119[tons/day] 1405 852 H₂ [kmol/h] 3550 2151 0 1215 1398 832 3446 0 0 CO[kmol/h] 965 585 0 159 380 236 775 0 0 CO₂ [kmol/h] 509 409 0 280 201141 621 0 0 CH₄ [kmol/h] 113 69 0 69 45 9 122 0 0 N₂ [kmol/h] 5 3 0 3 22 7 0 0 H₂O [kmol/h] 5 3 0 2 2 0 4 0 0 ζ  — 2.064 2.064 2.129 2.0641.837 2.023 CH₂OH [kmol/h] 0 0 453 1 0 0 1 1302 1755 [tons/day] 348 1 11000 1348

What is claimed is:
 1. A process for methanol synthesis from hydrogen,carbon monoxide and carbon dioxide under pressure, comprising chargingdesulphurized natural gas in a primary reformer and synthesis gas issubsequently admitted to a methanol synthesis; after passing through theprimary and a secondary reformer, supplying a side stream from thesynthesis gas stream to a methanol pre-reactor; supplying the methanolproduced in the pre-reactor to the methanol stream leaving from the mainstream of the methanol synthesis; and recycling a synthesis gas streamnot reacted in the methanol pre-reactor into the main stream prior tothe methanol synthesis; whereby an additional synthesis gas compensatingthe loss incurred is simultaneously charged within the zone where saidunreacted synthesis gas stream is recycled.
 2. The process according toclaim 1, comprising using a foreign synthesis gas stream from a separatesynthesis gas source or a synthesis gas originating from the natural gasstream charged as a bypass and passed via an “autotherm” reformer, ororiginating from another synthesis gas production as the additionalsynthesis gas.
 3. The process according to claim 1, comprisingwithdrawing the additional synthesis gas compensating the loss and beingsupplied to the main synthesis stream prior to the methanol synthesisfrom a combined “autotherm” reformer and/or withdrawing from a reactorfor partial oxidation or withdrawing from another synthesis gasproduction, the additional synthesis gas.
 4. The process according toclaim 1, comprising using off-heat of an additional methanol pre-reactorfor operating an absorption refrigeration machine; said machineproducing cooling used for cooling a synthesis gas compression and/orunused compression energy is used for compressing foreign synthesis gas.5. The process according to claim 1, comprising using a mixture of H²and CO₂ as the additional synthesis gas; and whereby the CO₂ present inthe mixture originates from flue gas of a firing operation or fromexhaust gas of a CO₂ washing operation in an ammonia plant.
 6. Theprocess according to claim 5, comprising the CO₂ originates from theflue gas purification plant of a firing operation, or the firing of theprimary reformer.